As an image sensor used in a digital still camera, a digital video camera, a camera for mobile phones, a camera for endoscopy and the like, a solid-state imaging device (so-called CCD sensor or CMOS sensor) is widely known in which pixels including photodiodes are arranged on a semiconductor substrate such as a silicon chip, and signal charges corresponding to photoelectrons generated in the photodiodes of each of the pixels are obtained in a CCD-type or CMOS-type reading circuit.
In the solid-state imaging device, in addition to the photodiodes, signal reading circuit and accompanying wiring thereof are formed in each of the pixels on the semiconductor substrate. Accordingly, as the pixels become miniaturized, the region of the signal reading circuit or the wire occupied in each single pixel is relatively widened, causing a problem of “reduction in aperture ratio” in which a light receiving area of the photodiode is reduced. The reduction in aperture ratio leads to a reduction in sensitivity.
Accordingly, Patent Document 1 proposes “a stack type solid-state imaging device” in which an aperture ratio is improved by stacking a photoelectric conversion layer on an upper side of a semiconductor substrate where each signal reading circuit and wiring are formed. For example, the constitution is such that a plurality of photoelectric conversion devices including a pixel electrode formed on the semiconductor substrate, a photoelectric conversion layer formed on the pixel electrode, and a counter electrode formed on the photoelectric conversion layer are arranged on a surface in parallel to the semiconductor substrate. In the photoelectric conversion device, an exciton generated in the photoelectric conversion layer according to the quantity of incident light is dissociated into an electron and a hole by applying a bias voltage between the pixel electrode and the counter electrode, and signals according to electric charges of the electrons or the holes that move to the pixel electrode according to the bias voltage are read out to the outside by a CCD-type or CMOS-type signal reading circuit installed on the semiconductor substrate.
The photoelectric conversion device is a device that generates electric charges in a photoelectric conversion layer in accordance with light incident from the side of a transparent electrode having light transmittance of a pair of electrodes, and reads out the generated electric charges as signal electric charges from the electrode. Exemplary photoelectric conversion devices are disclosed in Patent Documents 1 to 4. Further, Patent Documents 2 and 3 discloses a constitution using the photoelectric conversion layer formed of the organic semiconductor. Since the photoelectric conversion layer constituted by the organic semiconductor has a large absorption coefficient, the photoelectric conversion layer can be made thin, such that it is possible to realize a photoelectric conversion device in which electric charge diffusion to adjacent pixels rarely occurs, and optical color mixing and electric color mixing (crosstalk) can be reduced. A transparent conductive oxide manufactured on a glass substrate is used as a lower electrode.
However, the photoelectric conversion devices including a P-type organic semiconductor and an N-type organic semiconductor described in Patent Documents 1, 2 and 3 have a problem in view of durability because the device is likely to be structurally deteriorated, and need to be improved because sensitivity is deteriorated caused by the light irradiation.
In order to solve the above problems, Patent Document 4 discloses a stack structure of a fullerene deposited film and a fullerene polymerized film formed between two electrodes, in which deterioration of the film structure due to an unstable bond caused by intermolecular force of C60 molecules of the fullerene deposited film is overcome by the fullerene polymerized film. By using the characteristics of a thin film of fullerene, a sensor having improved physical and chemical stability is provided, while maintaining a performance operable at room temperature with improved durability.
However, from the present inventors' review, it is obvious that deterioration of sensitivity caused by the light irradiation cannot be suppressed by only using the method of Patent Document 4.
In addition, the solid-state imaging device is described as an example, but the problems of sensitivity deterioration equally occur even in a photoelectric conversion device used in a solar cell and the like.